Epidermal growth factor receptor (EGFR) is a trans-membrane receptor encoded by the c-erbB1 proto-oncogene with a molecular weight of approximately 170 kDa. EGFR is normally expressed in a wide variety of epithelial tissues as well as in the central nervous system. Accumulating evidence suggests that the level of EGFR overexpression is an important factor that directly correlates with active proliferation of malignant cells and poor prognosis of patients, thus, providing the rationale for the development of EGFR antagonists as potentially useful therapeutic strategies for the treatment of EGFR-expressing cancers.
EGFR inhibitors encompassing both small molecules and antibodies have been developed for the treatment of cancer. The small-molecule EGFR tyrosine kinase inhibitors (TKI) erlotinib (Tarceva®) and gefitinib (Iressa®) have demonstrated activity in multiple epithelial tumor types. These compounds reversibly bind to the adenosine triphosphate binding site of the EGFR TKD and inhibit autophosphorylation. Initial results with these molecules as monotherapy or in combination with chemotherapy in unselected populations were disappointing. It is now know that mutations in the EGFR gene alter the tumor phenotype and predict response to treatment, allowing the molecular selection of a subset of patients in which TKI are highly efficacious. The anti-EGFR monoclonal antibodies (mAbs) cetuximab (Erbitux®) and panitumumab (Vectibix®) are established agents in the treatment of CRC (colon and rectal cancer) and SCCHN (Squamous Cell Carcinoma of the Head and Neck). These agents have demonstrated modest clinical efficacy in combination with chemotherapy in phase III trials. However, patients with CRC with KRAS mutations (30%-40% of patients) are unresponsive to cetuximab or panitumumab, when used as monotherapy or in combination with chemotherapy. mAbs targeting cell surface receptors can exert a therapeutic effect either by inhibiting the oncogenic growth signal (blocking ligand binding and/or receptor dimerisation/activation) or through direct cell killing. Cell killing can be achieved by inducing apoptosis in the target cell or cell killing can be achieved by releasing cytotoxic compounds in the target cell through antibody-drug conjugates (ADCs), which consist of cytotoxic agents or toxins chemically conjugated to a monoclonal antibody. Antibody-drug conjugates potentially represent an advantage over treatment with chemotherapy because they are designed to deliver the cytotoxic agent specifically to tumor cells thereby resulting in an improved safety profile.
Maytansinoids are highly cytotoxic compounds which inhibit the formation of microtubule protein polymerization (Remillard, et al., Science 189, 1002-1005 (1975)). Maytansine was first isolated by Kupchan et al. (J. Am. Chem. Sci 94:1354-1356 (1972)) from the east African shrub Maytenus serrata. Maytansinoids including maytansinol and C-3 esters of maytansinol were also produced by certain microbes (U.S. Pat. No. 4,151,042). Various analogues of maytansinol with different cytotoxicity have also been prepared by synthetic chemistry (for review see Chem. Pharm. Bull. 52(1) 1-26 (2004)). Examples of mytansinoids include maytansine, mertansine (MD1), MD3 and MD4. Maytansine is a strong mitotic inhibitor and shows significant inhibitory activity against multiple tumors including Lewis lung carcinoma and B-16 melanocarcinoma solid murine tumor models. Maytansine was reported to inhibit the human acute lymphoblastic leukemia line C.E.M. at concentrations as low as 10−7 mg/mL (Wolpert-DeFillippes et al., Biochem. Pharmacol. 1735-1738 (1975)). It also showed to be 100- to 1000-fold more cytotoxic than conventional chemotherapeutic agents like methotrexate, daunorubicin, and vincristine (U.S. Pat. No. 3,896,111).
Ansamitocins, the bacterial maytansinoids, show an activity spectrum and effective dosage range similar to maytansine. They inhibit P388 leukemia at daily doses as low as 0.8 μg/kg. Ansamitocin P3 (AP3) was also shown to be effective against multiple cancer cell lines (for review see Alkaloids, vol. 2, 149-204 (1984); Chem. Pharm. Bull. 52(1) 1-26 (2004)). The maytansinol C-3 esters with N-methyl-L-alanine derivatives are found to be much more cytotoxic than the corresponding esters of simple carboxylic acid and to be 100 times more cytotoxic than their epimers corresponding to N-methyl-D-alanine (U.S. Pat. Nos. 4,137,230; 4,260,608; Kawai, et al., Chem. Pharm. Bull. 32: 3441-3451 (1984); Widdison, et al., J. Med. Chem. 49: 4392-4408 (2006)).
Maytansinoids were expected to have the capacity to treat many different cancers due to their highly toxic nature and the in vitro activities against multiple cancer cell lines. However, the toxicity also made this class of compounds not favorable in human clinical trials as the side effects were intolerable for many patients (Issel et al., 5 Cancer Treat. Rev. 199-207 (1978)). Accordingly, targeted delivery of cytotoxic compounds to cancer cells by conjugating toxic drugs to monoclonal antibodies (ADC for antibody drug conjugate) is proposed in order to reduce the side effects. Certain conjugates of cytotoxic drugs such as maytansinoids, auristatins, anthracyclins, duocarmycins, etc. with antibodies are being evaluated in preclinical or clinical studies in the treatment of diseases.
Antibody drug conjugates (ADCs) are composed of three key elements: antibody, linker, and drug. The selection of a particular antibody and drug will have a great impact on the efficacy and safety depending on the particular disease. Linker stability and the method by which the drug is conjugated to the antibody plays a critical role in the success or failure of the ADC drug development.
The efficacy of an ADC depends in part on combination of a variety of parameters, involving not only the specificity of the antibody and the potency of drugs, but also the linker's stability or sensitivity to cleavage, the cell surface triggered the internalization, trafficking, and subsequent release of the active cytotoxic payload. Thus, ADC comprising different drug linkers or with different antibodies against the same target can vary significantly in their utility.